rails dynamic nested form data select - ruby-on-rails

Question

Rails gurus, do you know of a standard solution to this problem I've been struggling with?
In my app, the user can define properties for his objects. So before generating his list of objects (let's say they are books), he can specify which properties he cares about and their potential values, and then for each book he will have to input a legal value for each property. So say I put in for my properties: length (legal values "long", "short") and difficulty ("easy", "hard"). On a different bookshelf, a different list of books could have different properties (cover_color "blue" or "red")
So now I am in my book form. "Add new book on this bookshelf." On the partial, I come up with the list of properties relevant to a new book on this bookshelf (length, and difficulty). Then I look up the legal values for each property. I have a select dropdown in which the user can choose one:
<% for prop in #book.properties %>
<%= prop %> :
<%= f.collection_select :prop_value_select, prop.legal_property_values, :id, :name %>
<%end %>
In my book model, I defined a virtual attribute that will create the "join record" PropertyValue. PropertyValue has legal_property_value_id and book_id.
def prop_value_select=(incoming_id_from_form)
PropertyValue.create!(:legal_property_value_id => incoming_id_from_form, :book=> self)
end
The whole scheme works, except my method is only getting called once, for the first property when the form submits, instead of once for each property.
I am racking my brain... seems very simple, but what's the standard rails way to do something like this?
collect all of the properties into an array and generate the models as a callback?
some magic with a partial for each property?
Thank you!

Answer 1

I think the problem is that you are using a nested model, and all your fields will have the same id's, so rails will only store one nested element.
You should be using fields_for which allows you to handle a nested model.
First you need to specify in your model that it will accept nested attributes. Add the following line
class Book
has_many :properties
accept_nested_attributes_for :properties
end
and in your view you would write something like
<% fields_for :properties do |prop| %>
<%= prop %> :
<%= f.collection_select ...
<% end %>
Hope this helps.